1. Field of the Invention
The present invention relates to an optical amplifying repeater which forms the central part of, for example, a submarine optical amplifying and repeating apparatus.
At present, in long-distance transmission of optical communications systems, the so-called optical amplifying and repeating systems are coming into wide use, in which systems optical amplifiers are contained in the above repeaters.
In such optical communications systems, because the distance from one end office to another is extremely long, it is not easy to monitor or control each of the optical amplifying repeaters which are cascade connected between the two end offices.
For this reason, in general, to perform monitoring or control of each of the optical amplifying repeaters, a remote repeater monitor and control system, which uses light modulation, is employed. The present invention includes an improvement to an optical amplifying repeater which is managed by a remote repeater monitor and control system.
2. Description of the Related Art
As will be described in detail with regard to the accompanying drawings, in a multi-stage optical amplifying and repeating transmission system, a long-distance optical transmission line is laid between two end offices A and B, a plurality of optical amplifying repeaters being connected in cascade along this optical transmission line.
Assume that a cable failure occurs when data is being transmitted from end office B to end office A. When this occurs, because the normal reception of data at the end office A is suspended, end office A immediately starts a search for the causative fault. This fault search is performed by the use of a command signal CM by which a main (carrier) optical signal S is modulated and the response signal RS which is output from each amplifying repeater in response thereto.
In the above-noted system, when the above-noted cable failure occurs, the optical carrier signal S is no longer input to the optical amplifying repeater. This is a loss of the optical input.
When this optical input loss occurs, because the optical carrier signal S is no longer input to the optical amplifying repeater, it becomes impossible to apply the modulation of the above-noted response signal RS to this optical carrier signal S. When this occurs, this optical amplifying repeater applies RS modulation instead to the ASE (amplified spontaneous emission). This ASE is a noise component of a relatively wide bandwidth which is inevitably generated by each optical amplifying repeater.
Although it is theoretically possible to apply modulation in accordance with the response signal RS to this ASE, in actuality the modulation of the amplified spontaneous emission by the response signal RS is insufficient. As will be described later, in the range in which the drive current I.sub.LD of the optical amplifying repeater is small, the variation of the amplified spontaneous emission with respect to the deviation in I.sub.LD is relatively large.
In response to the loss of optical input, when the ALC (automatic level control) loop operates sufficiently, so that drive current I.sub.LD of the optical amplifier becomes large, in this range in which I.sub.LD is large, the variation in the output of the amplified spontaneous emission is relatively small. For this reason, when applying modulation to the ASE with the amplitude of the normal response signal RS, the resulting modulation output is extremely small. Ultimately, after the optical carrier signal S is lost, even if modulation is applied to the ASE by the response signal RS, it becomes difficult to perform such modulation. As a result, it becomes difficult to receive the response signal RS at the end office and, in the worst case, it becomes impossible to monitor the optical amplifying repeater, and impossible to perform troubleshooting of the fault point.